Meiosis

Why meiosis is needed

Body (somatic) cells are diploid — they contain two sets of chromosomes (2n), one set inherited from each parent, arranged in homologous pairs. If sex cells (gametes) were also diploid, fertilisation would double the chromosome number every generation.

Meiosis solves this problem: it is a type of cell division that halves the chromosome number, producing haploid (n) gametes — sperm and egg cells in animals. When a haploid sperm fertilises a haploid egg, the diploid number is restored in the offspring.

Meiosis I — homologous pairs separate

Meiosis I is the "reduction division", where chromosome number is halved:

• Homologous chromosomes (one from each parent) pair up tightly — this is called synapsis, forming a structure of four chromatids (a bivalent/tetrad).
• While paired, non-sister chromatids can exchange segments of DNA in a process called crossing over, creating new combinations of alleles — a major source of genetic variation. The X-shaped point where the chromatids overlap and exchange material is called a chiasma (plural: chiasmata).
• The homologous pairs then separate, with one whole chromosome (still as two sister chromatids) from each pair moving to opposite poles.

The independent way each homologous pair lines up and separates (independent assortment) adds even more genetic variation between gametes.

Meiosis II — sister chromatids separate

Meiosis II happens in both resulting cells and closely resembles mitosis: the sister chromatids of each chromosome separate and move to opposite poles. No further DNA copying happens between Meiosis I and II.

The result: four genetically varied gametes

Meiosis produces four haploid daughter cells, each genetically different from one another and from the parent cell — due to crossing over and independent assortment. This genetic variation is the raw material for evolution by natural selection, and is why offspring resemble but are never identical to their parents (except identical twins).

Karyotypes and gonosomes

A karyotype is a representation (often a photograph) of the number, shape, and arrangement of all the chromosomes in a cell's nucleus, usually arranged in matching homologous pairs from largest to smallest. In humans, a normal karyotype shows 23 pairs of chromosomes: 22 pairs of autosomes (non-sex chromosomes) plus one pair of gonosomes — the sex chromosomes (XX in females, XY in males). Karyotypes are used to detect chromosomal abnormalities, such as an extra or missing chromosome resulting from non-disjunction during meiosis.

Mitosis vs meiosis

Polyploidy

Polyploidy occurs when an organism has more than two complete sets of chromosomes (more than the normal diploid number) — for example 3n (triploid) or 4n (tetraploid), instead of the usual 2n. It typically arises from an error in meiosis or fertilisation that fails to properly halve or combine chromosome sets.

Polyploidy is common and often beneficial in plants (many crop and wild flowering-plant species are polyploid), though rare and usually harmful in animals. Polyploid plants often show useful traits linked to their extra genetic material:

• Larger cells, often producing larger seeds, fruit, or flowers.
• Sometimes increased seed or flower number.
• Greater genetic buffering — with extra gene copies, a harmful mutation in one copy is more easily masked, which can help a polyploid population cope with a changing environment.